1. Field of the Invention
The invention relates to display systems for computers, and more particularly to display systems having graphics modes of operation.
2. Description of the Related Art
Personal computers are equipped with display systems so that information can be provided to the user. Typically the display systems can operate in character or graphics mode and can be used with monochrome or color displays. The display system commonly has a buffer, called the video buffer, which is addressable by the processor in the computer and which contains values relating to the information to be displayed. These values are provided to circuitry which obtains these values and properly converts them for control of the display, conventionally a cathode ray tube (CRT) or a liquid crystal display (LCD).
In text or character mode of operation the values written to the video buffer are in the form of encoded values representing entire characters, such as ASCII. The circuitry utilizes these character values and stored font information to develop a pixel data stream for presentation to the actual display device. The circuitry typically obtains the character value for a given display location and obtains the necessary pixel information for a given scan line from a font generator. These pixel values are then transmitted to the display driver circuitry. Because the ASCII values, for example, are stored in the video buffer it is quite easy to write and read the character values at a given display position. The display location is converted to the proper address in the video buffer and a write or read operation performed.
This situation is complicated in graphics mode, also referred to as all points addressable (APA) mode. In this mode the on/off state or color of each pixel is individually determined by the contents of a corresponding value, a single bit or a series of bits, in the video buffer. Thus, characters are developed on the display by use of a group of pixels, for example 64 pixels in an 8.times.8 array, each pixel being set to a desired state to define the character. To complicate matters each pixel may have attributes, commonly color values when in graphics mode, associated with it. Therefore a write operation is not a simple task of writing a character value to a single location in the video buffer, but involves writing the proper values to each pixel forming the group or array.
Reading a character value is even more difficult in that the pixel values in the array must be matched to a preset font array to determine the character actually present at a given location. This matching involves a very large number of processor operations and as a result is quite slow.
One technique for performing the write and read operations of characters when a display system is in graphics mode is shown in U.S. Pat. No. 4,408,200 to Bradley, reissued as Re. 32,201. This patent disclosed using a dot image or mask of the desired character which was stored in a read only memory (ROM). The dot image contained a compressed value of the actual information to be placed in the video buffer. The compressed values were either 0 where a background color was to be used or a 1 where a preselected color was to be used. Each character in the ROM was formed by 8 bytes, while the corresponding values in the video buffer were 8 words, that is, twice the size of the compressed values. The compressed dot image values were expanded by utilizing two bits for each bit in the compressed format, with the two bits indicating the desired color of the pixel, either the background for a 00 value or one of three color values if a non-00 value was present. This expansion was done by analyzing the dot image single bits and converting them to the corresponding double bits. This technique was not wholly satisfactory for operation because the processor was required to do each of the expansions for each pixel in each character, thus using large amounts of processor time.
The read operations were likewise slow and difficult. Because the video buffer values did not correspond directly to character values, compression was required for each pixel in each character desired, which then had to be matched against the dot image information available f rom the font table. Very large numbers of processor, memory and slow video buffer accesses were required to perform the operation.